How to add a panel to the properties region in the Image Editor?
The script works as written. The "problem" with the script is that it is putting the hello world in the text editor and not the image editor. bl_space_type takes:

EMPTY, VIEW_3D, GRAPH_EDITOR, OUTLINER, PROPERTIES, FILE_BROWSER, IMAGE_EDITOR, INFO, SEQUENCE_EDITOR, TEXT_EDITOR, DOPESHEET_EDITOR, NLA_EDITOR, TIMELINE, NODE_EDITOR, LOGIC_EDITOR, CONSOLE, USER_PREFERENCES

as accepted values, one for each window type in blender. The script in question has TEXT_EDITOR and not IMAGE_EDITOR.

import bpy

class HelloWorld(bpy.types.Panel):
    """Creates a Panel in the Image Editor properties region"""
    bl_label = "Hello World"
    bl_idname = "OBJECT_PT_hello"
    bl_space_type = 'IMAGE_EDITOR'
    bl_region_type = 'UI'

    def draw(self, context):
        layout = self.layout
        obj = context.object

        row = layout.row()
        row.label(text="Hello world in the image editor!", icon='WORLD_DATA')

def register():
    bpy.utils.register_class(HelloWorld)

def unregister():
    bpy.utils.unregister_class(HelloWorld)

if __name__ == "__main__":
    register()